1. Field of the Invention
The present invention relates to a display apparatus using an organic electro-luminescent element and a manufacturing method thereof. In particular, the invention relates to a technique for improving a display apparatus using an organic electro-luminescent element, emitted light of which is taken out of its upper surface on its cathode side, to have a larger aperture rate.
2. Description of the Related Art
An organic electro-luminescent element may be utilized as a pixel of, for example, an active matrix type display apparatus. An organic electro-luminescent display apparatus using an organic electro-luminescent element as its pixel is regarded as a promising next generation flat panel display apparatus in place of a liquid crystal display apparatus.
FIG. 7 shows a structure of a conventional organic electro-luminescent element. In FIG. 7, on a glass substrate 1, there is formed an anode A of a transparent conducting film such as ITO (Indium Tin Oxide). An organic layer 10 is laminated on the anode A. Then, a cathode K made from a metal is formed on the organic layer 10. Thereby, the organic electro-luminescent element having a diode structure can be obtained.
The cathode K is made from, for example, an alloy of aluminum and silver or an alloy of magnesium and silver. The thickness of the cathode K is about 100 nm, for example.
The organic layer 10 is basically made by laminating a hole transporting layer HTL, a luminescent layer LUL and an electron transporting layer ETL on the anode A in the order.
In such a structure, when electrons and holes are injected from the cathode K and the anode A, respectively, the injected electrons and holes pass through the electron transporting layer ETL and the hole transporting layer HTL, respectively, and then they are recombined at the luminescent layer LUL to emit light.
In this case, the emitted light is taken out of the glass substrate 1 side. That is, the structure of the OLED is the so-called downside light taking out structure. The luminous element made by sandwiching the organic layer 10 between the cathode K and the anode A like this becomes an organic light emitting diode (OLED).
Because the response speed of the OLED as an organic light emitting diode is 1μ sec. or less, it is possible to perform the time division duty drive of the OLED arranged in a simple matrix form when the OLED is applied to a display apparatus. However, when the OLED comes to have high duty with the increase of its pixels, it is necessary to supply an instantaneous large current to the OLED for securing enough brightness.
On the other hand in an active matrix type display system, because a signal voltage is kept by holding capacitance formed between the OLED and a thin film transistor at each pixel during one frame interval, a drive current can be imposed on the OLED in accordance with the signal voltage. Consequently, it is not necessary to supply the instantaneous large current like in the case of the simple matrix system, and thereby the OLED is scarcely damaged.
However, when a panel display apparatus is designed by means of the active matrix system using a switching element of a thin film transistor, the thin film transistor is formed by being laminated on the glass substrate 1. Consequently, an opened area for taking light out of the OLED is narrowed by the thin film transistor formed on the glass substrate 1 in case of the downside light taking out structure shown in FIG. 7. Therefore, the downside light taking out structure has a problem that the aperture rate thereof is reduced, wherein the aperture rate is defined by dividing an effective display area with unit pixel area.
For avoiding the problem, the so-called upside light taking out structure where emitted light is taken out of the cathode K disposed on the upper side of the glass substrate 1 is available.
The upside light taking out structure is shown in FIG. 8. As shown in FIG. 8, a reflecting layer 110 is formed on the glass substrate 1. An anode A composed of a transparent conducting film such as ITO is formed on the reflecting layer 110. An organic layer 10 is superposed on the anode A. And then, a metal layer 11 is formed on the organic layer 10. In this case, the thickness of the metal layer 11 is 10 nm or less for making it possible that emitted light penetrates the metal layer 11. A transparent conducting layer 12 such as ITO is formed on the metal layer 11.
Because the emitted light is taken out of the upside, the cathode K is made of a metal foil film having a low work function so that its transmittivity is high and electrons can effectively be injected. For example, the cathode K is made by forming an alloy of aluminum and lithium to be a thin film of 10 nm in thickness or forming an alloy of magnesium and silver to be a thin film of 10 nm in thickness.
A transparent conducting layer 12 is further formed on the metal layer 11 to be a thickness, for example 100 nm. The transparent conducting layer 12 performs a role of a protection of the thin metal layer 11 and a role of changing the resistance of wiring to be low.
In such a device structure of the upside light taking out structure, the numerical aperture of a pixel can principally be enlarged in comparison with the device structure of the downside light taking out structure. However, when a display apparatus is composed by arranging devices of the upside light taking out structure in a matrix form as its pixels, there is often arranged a structure such as a barrier plate for interrupting light on the front face side of the substrate 1. Accordingly, it is urgently necessary to develop a mounting structure capable of realizing a larger aperture rate.